A conventional spectrophotometer such as an ultraviolet visible spectrophotometer which has been widely and generally used includes an optical system. In the optical system, for example, a light emitted from a light source is wavelength-dispersed by a spectroscope, a measurement light having a specific wavelength is taken out, the measurement light is delivered onto or into the sample placed in a sample chamber, and the light which has passed through the sample is detected. In the sample chamber, various types of sample cells and sample change mechanisms are placed in accordance with the purpose of analysis, the kind of sample, and other factors (for example, refer to Patent Document 1 or other documents). For example, in the measurement of the liquid sample's transmission characteristics such as transmissivity and absorbance, a polygonal or cylindrical cuvette cell for holding a liquid sample is generally used. The interior volume of a general cuvette cell is more than a few mL and it is required to prepare a sufficient quantity of liquid sample to fill the cuvette cell.
In recent years, an ultraviolet visible spectrophotometer as previously described has been used in the field of biochemistry, such as quantifying protein and DNA. In such cases, the quantity of the liquid sample to be analyzed is usually extremely small. In particular, in a DNA-related analysis, a sample is precious and expensive. In some cases, it is necessary to perform an analysis with a liquid sample of less than a few μL. A cuvette cell as previously described cannot be used for the purpose of analyzing such a small amount of liquid sample. In this connection, a container suitable for spectro-analyzing such a small amount of liquid sample is conventionally known.
Patent Document 2 or other documents for example disclose a sample cell for measuring a trace liquid sample. This is a capillary cell which siphons and holds a liquid sample by using a capillary action. However, even such a capillary cell generally requires a liquid amount of more than a few μL and a liquid sample less than this amount cannot be analyzed. In addition, a capillary cell has disadvantages in that injecting a liquid sample into the cell is cumbersome and cleaning after a measurement is troublesome.
On the other hand, as an apparatus capable of spectro-analyzing an extremely small amount (approximately 1 μL) of liquid sample, a spectrophotometer ND-1000 which is sold by NanoDrop Technologies Inc. in the United States is recognized (refer to Non-Patent Document 1). The schematic configuration of the sample holding unit in this spectrophotometer is illustrated in FIG. 9. In the sample holding unit, the downward end face of a light-delivering optical fiber 41 held by an upper base 40 and the upward end face of a light-receiving optical fiber 43 held by a lower base 42 are placed in such a manner as to face each other in the vertical direction. The lower base 42 is immobile, whereas the upper base 40 is vertically movable.
In placing a liquid sample, the upper base 40 is moved upwards for example and the liquid sample is dropped onto the upward end of the light-receiving optical fiber 43. After that, the upper base 40 is moved once downwards to the position where the downward end face of the light-delivering optical fiber 41 almost touches the upward end face of the light-receiving optical fiber 43 (refer to FIG. 9(a)). Then, the upper base 40 is drawn up to a predetermined position. This makes the liquid sample S vertically bridge the space between the downward end face of the light-delivering optical fiber 41 and the upward end face of the light-receiving optical fiber 43, forming a catenoid shape due to the surface tension, as illustrated in FIG. 9(b).
In this state, the liquid sample S between the light-delivering optical fiber 41 and the light-receiving optical fiber 43 can serve as an optical connector. Accordingly, the measurement light which has been delivered through the light-delivering optical fiber 41 passes through the liquid sample S to be sent into the light-receiving optical fiber 43. Generally, the optical path length in a liquid sample is set to be approximately 1 mm, and a very small amount (i.e. approximately 1 through 2 μL) of liquid sample can be analyzed.
However, this spectrophotometer has a disadvantage in that a light quantity loss occurs unless the optical axis of the light-delivering optical fiber 41 is coincident with that of the light-receiving optical fiber 43. Hence, the accuracy of the mechanism for vertically moving the upper base 40 and the positional accuracy of both the bases 40 and 42 are required to be sufficiently high, which increases the cost by that much. In addition, since the optical fibers 41 and 43 constituting the measurement optical system directly contact with the liquid sample S, the sample change operation is far more troublesome and time-consuming compared to the case, for example, of inserting a sample cell in a measurement light path in a sample chamber space or the like as previously described. Above all, it takes time to perform the measurement of a great number of samples while automatically changing the samples.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. H05-315324
[Patent Document 2] Japanese Unexamined Patent Application Publication No. H05-302893
[Non-Patent Document 1] “NanoDrop ND-1000 Overview,” [online], NanoDrop Technologies Inc., [Sep. 25, 2006], internet <URL: http://www.nanodrop.com/nd-1000-overview.html>